This invention relates generally to magnetrons of a type using rotating cylindrical sputtering targets, and, more specifically, to structures and techniques for minimizing arcing in such magnetrons.
Cylindrical magnetrons are becoming widely used for depositing films on substrates. An example is the deposition of a stack of dielectric and metal layers on a surface of a glass substrate for the purpose of filtering out a portion of solar energy from passing through the glass. Such a substrate is positioned within a vacuum chamber containing at least one, and usually two, rotating cylindrical targets containing sputtering material on an outer surface thereof. Both inert and reactive gases are generally introduced into the chamber. A voltage applied to the sputtering target, with respect to either the vacuum chamber enclosure or a separate anode, creates a plasma that is localized along a sputtering zone of the target by stationary magnets positioned within the target. Material is sputtered off the target surface and onto the substrate by bombarding the target with electrons and ions of the plasma as it passes through the stationary sputtering zone.
The magnets are usually of a permanent magnet type, arranged along a line within the rotating cylindrical target and held against rotation with the target. The sputtering zone is created by the magnets along substantially the entire length of the cylindrical sputtering target and extends only a small circumferential (radial) distance around it. Traditionally, the magnets are arranged so that the sputtering zone exists at the bottom of the cylindrical target, facing a substrate being coated directly beneath.
Although deposition of the film is desired to take place only on the substrate, it is also deposited on other surfaces within the reactive chamber. This can create a problem in many situations, especially when certain dielectrics are being deposited as the film. For example, if the target surface is silicon and the reactive gas is oxygen, silicon or aluminum oxides are deposited on the target surface, surfaces of target supporting structures, and the like, as well as on the substrate that is intended to be coated. After a certain build-up of dielectric material on internal vacuum chamber surfaces has occurred over time, arcing to those surfaces can begin. Arcing is undesirable since it generates particles that contaminate the film being deposited on the substrate, and overloads the power supply that creates the plasma through an electrical connection with the sputtering target surface and the vacuum chamber walls or some other anode.
An advantage of a rotating cylindrical sputtering target is that such a film deposited on the target is subjected to being sputtered away as the target surface passes through the sputtering zone, thus counteracting the undesirable film build-up. Despite this self-cleaning characteristic, however, undesirable arcing still occurs in rotary magnetrons under certain circumstances.
A cylindrical magnetron shield structure has been developed to minimize this undesirable arcing that occurs in rotary cylindrical magnetrons. See Kirs et al., "Cylindrical Magnetron Shield Structure," U.S. Pat. No. 5,108,574 (hereinafter, Kirs et al.), which is incorporated herein by this reference. As shown in Kirs et al., the deposition of dielectric film can be minimized by dark space shielding, which prevents plasma formation in the dark space and thereby reduces film deposition and subsequent arcing.
Although the shield structure of Kirs et al. greatly enhances the self-cleaning characteristic of rotary cylindrical magnetrons, some deposition of condensate has been found to occur at the far ends of the target cylinder. Unlike the deposition of dielectric films that concerned Kirs et al., this deposition of condensate from the vapor present in the system occurs regardless of the existence of plasma. Thus, the problem of condensate deposition is not fully resolved by the use of dark space shielding.
Recently, a cylindrical magnetron shield structure has been developed to minimize the occurrence of such condensation deposition and related arcing at the ends of the target cylinder. See Sieck et al., "Cylindrical Magnetron Shield Structure," U.S. patent application Ser. No. 08/308,828, a continuation of U.S. patent application Ser. No. 08/004,964 (hereinafter, Sieck et al.), which is incorporated herein by this reference. Particularly, the shield structure is shaped at its inner edges to conform substantially to a pattern of condensation that results when the magnetron is operated with the target held stationary. The shaped shield structure is positioned with respect to the target to shield regions where condensation of vaporized material would otherwise occur at a rate which exceeds the rate at which deposited condensate is removed from the target by sputtering. Use of the shaped shield structure of Sieck et al. greatly reduces condensate deposition at the ends of the target and related arcing activity.
However, even slight deposition of dielectric or insulating material at the ends of the target may result in undesirable arcing activity. Generally, when the condensate build-up approaches ground or vacuum chamber potential, arcs to the condensate surface appear to travel across the condensate surface toward the vacuum chamber wall or other anode.
Additionally, prior to the present invention, rotatable magnetrons have been configured such that the shield structure is at ground, vacuum chamber potential or cathode potential. In these magnetrons, the shield structure has proven to be a site of preferential arcing from the cathode to the vacuum chamber wall or other anode.
It is a principal object of the present invention to provide a mechanism and technique for minimizing such undesirable arcing activity.